The present invention relates generally to optical systems and, more particularly, to a method and apparatus for performing optical coherence tomography.
Optical coherence tomography or OCT is an imaging technique used to provide cross-sectional images of biological systems. OCT has been used to diagnose and monitor ocular diseases such as glaucoma and macular edema and, to the extent permissible by optical attenuation, image non-transparent tissues. In vitro OCT has also found applications in arterial pathology, gastroenterology, urology, and neurosurgery. An advantage of OCT over ultrasound imaging is its ability to achieve spatial resolutions of 10 micrometers or less, approximately 10 times better than that offered by ultrasound.
In operation, a beam of light is focussed on the tissue to be imaged and the tissue reflectivity as a function of depth is measured using a scanning interferometer. By scanning the light beam in a transverse direction, a cross-sectional image is constructed. As the axial resolution is directly proportional to the coherence length of the light source, and the coherence length is inversely proportional to the spectral bandwidth, typically either mode-locked lasers or semiconductor sources with a chirped quantum well structure are used. For example, OCT using a Kerr-lens mode-locked Ti:Al2O3 laser is described in an article by Bouma et al. entitled High-Resolution Optical Coherence Tomographic Imaging Using a Mode-Locked Ti:Al2O3 Laser Source published in Optics Letters, Vol. 20, No. 13, Jul. 1, 1995.
Conventional OCT light sources suffer from various limitations. Chirped semiconductor lasers used in these systems often have inadequate brightness to achieve the desired OCT image contrast and frequently provide too little bandwidth to achieve the desired resolution. Alternative sources based on mode-locked solid state lasers are large, costly, and can require frequent adjustment. Furthermore, due to the bandwidth inflexibility of the employed sources, an OCT system designed for one type of biological tissue may not be optimal for another type of biological tissue. Accordingly, what is needed in the art is a low cost, compact OCT system utilizing a high brightness source with a large, preferably adjustable, bandwidth. The present invention provides such a system.
The present invention provides a method and apparatus for performing optical coherence tomography using a wavelength multiplexed source in which the single output beam is of a large bandwidth. As a result of the large bandwidth, the tomography system achieves very high resolution, thus allowing cellular structures to be resolved. In order to achieve high contrast as well, the wavelength multiplexed source has minimal wavelength separation between spectrally adjacent lasers and has an output beam with an approximately Gaussian spectral shape.
In at least one embodiment of the invention, a multi-gain element array is used within an external resonator. Interposed between the array and the resonator output coupler are a collimating element and a diffraction grating. Typically either a refractive optic or a reflective optic is used as the collimating element although for some applications a xc2xc pitch GRIN lens can be used. The diffraction grating can either be transmissive or reflective. The combination of the diffraction grating and the collimating element forces each emitter within the array to lase at a distinct wavelength. Since the gain bandwidth of a single emitter array is typically less than the desired bandwidth, either multiple arrays of differing center wavelength are packaged together or a large array is used with a laterally varying quantum well thickness or epitaxy. An intracavity spatial filter can be used to improve the beam quality and reduce cross-talk. External to the resonator cavity is an optical delivery system for transmitting the optical energy to the area to be imaged. If necessary, the spectral profile of the output beam can be improved using a dielectric filter. Alternately, the spectral profile of the output beam can be modified using a second diffraction grating and an additional optical element interposed between the resonator cavity and the optical delivery system. The grating redisperses the light in the output beam while the additional optical element reshapes the profile as desired.
In at least another embodiment of the invention, the outputs of a pair of multiple gain elements arrays are multiplexed within a single resonator cavity, the resonator cavity being comprised of a high reflector, preferably applied to the back facets of the arrays, and an output coupler. Multiplexing can be achieved, for example, with a polarization sensitive beam combiner. Interposed between each array and the output coupler are a collimating optic and a single diffraction grating, both of which can either be transmissive or reflective. The combination of the diffraction grating and the collimating element forces each emitter within each array to lase at a distinct wavelength. Each of the arrays are positioned relative to one another and to the diffraction grating in such a manner as to cause an interlacing of the lasing wavelengths of the individual gain elements of the two arrays. As a consequence, the wavelength separation between spectrally adjacent lasers is further reduced, thus achieving further improvement in image contrast. Each array can be comprised of multiple arrays of differing center wavelength packaged together or of a single, large array with a laterally varying quantum well thickness or epitaxy. An intracavity spatial filter can be used to improve the beam quality and reduce cross-talk. External to the resonator cavity is an optical delivery system for transmitting the optical energy to the area to be imaged. In order to improve the spectral profile of the output beam, either an optical filter or the combination of a second diffraction grating and an additional optical element can be interposed between the resonator cavity and the optical delivery system.